Chemistry definitions



Chemistry Terms/Definitions LC 2015Acid: Arrhenius: a substance that dissociates in water to produce H+ ions. Bronsted-Lowry: a proton [H+] donor. Activation energyThe minimum amount of energy which colliding molecules must have before they can react together.Addition polymerA polymer made by the combining together of small unsaturated molecules of the same compound.Addition reactionA reaction in which the double or triple bond of an unsaturated compound opens up and other atoms add on to the molecule. Addition reactions are characteristic of the alkenes and alkynes.Alkali metalsThe elements of Group 1 of the Periodic Table.AllotropesAllotropes are different physical forms of the same element (and are due to different arrangements of the atoms of the element). The allotropes of carbon are diamond, graphite and charcoal.Aliphatic Compounds An organic compound that consists of carbon atoms and closed chain compounds (rings) that resemble them in chemical properties.AlkaliA base that is soluble in water.Alkaline earth metalsThe elements of Group 2 of the Periodic Table.AlkanesHydrocarbons in which there are only single bonds between carbon atoms.AlkenesHydrocarbons in which there is a double bond between two carbon atoms.AlkynesHydrocarbons in which there is a triple bond between two carbon atoms.Amphoteric substanceOne which can act both as an acid and as a base.AnionThe negative ion that is attracted to the anode.AnodeThe positive electrode in electrolysis.Aromatic compounds Compounds that contain a benzene ring structure in their molecules.AtomSmallest particle of matter that can exist by itselfAtomic numberThe atomic number corresponds to the number of protons in the nucleus of an atom of that element. Atomic RadiusHalf the distance between 2 adjacent atoms of the same element joined by a single covalent bondAufbau principleElectrons occupy the lowest available energy level.Autocatalysis A product of the reaction catalyses the reaction. Mn2+ catalyses MnO41- and ethanolAvogadro constantThe number of carbon atoms in exactly 12 grams of the carbon-12 isotope.Avogadro’s lawEqual volumes of all gases at the same temperature and pressure contain equal numbers of molecules. 1 mole of any gas at STP occupies 22.4 LBase: Arrhenius: a substance that dissociates in water to produce OH- ions. Bronsted-Lowry: a proton [H+] acceptor.Biochemical oxygen demand (BOD)A measure of the amount of organic pollution in water. It is defined as the amount of dissolved oxygen consumed by biochemical action, when a sample of water is kept in the dark at 20° C for 5 days.Bond Energyis the energy required to break one mole of covalent bonds and to separate the neutral atoms completely from one anotherBoyle’s Law At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.Bronsted acidA substance (molecule or ion) that can donate protons.Br?nsted baseA substance that can accept protons.CatalysisThe process of changing the rate of a chemical reaction by the action of a catalyst.CatalystA substance that alters the rate of a chemical reaction but which is not used up during the reaction, and which is chemically unchanged at the end of it.Catalyst poisonA substance which destroys the activity of a catalyst.Catalytic CrackingThe breaking-down of long-chain hydrocarbon molecules into short-chain molecules for which there is greater demand.CathodeThe negative electrode in electrolysisCationThe ion which is attracted to the cathode.Charles’s LawAt constant pressure, the volume of a fixed mass of gas is directly proportional to its temperature measured on the Kelvin scale.ConcentrationThe amount of a substance per unit volume of solution.Condensation reactionA reaction in which two compounds combine together and eliminate water (or other small molecule such as HCl).Conjugate acidThe substance formed when a Br?nsted base has accepted a proton.Conjugate baseThe substance formed when a Br?nsted acid has lost a proton.Conjugate Acid-Base Pair Pair of compounds that differ by H+ - one with most H’s is acid – other is base e.g. H2O and H3O+Co-productIn an industrial process, a substance that is also formed along with the substance being manufactured.Covalent bondOne or more shared pairs of electrons, each of the bonded atoms contributing one electron towards the shared pair. Covalent bonds may be either pure or polar.CrackingThe breaking down of long-chain hydrocarbon molecules into smaller molecules.Dative BondCovalent bond where one atom supplies both electrons e.g. NH4+ or H3O+D-block elementOne whose atoms’ highest-energy electron occupies a d orbital.DehydrationThe removal of water, or the elements of water, from a compound or compounds.DehydrocyclisationA reaction in which an alkane is converted into an aromatic hydrocarbon and hydrogen. For example, Deionised waterWater in which ions that were present in it have been removed by passing the water through an ion exchanger, which replaces any positive ions by H+ ions and any negative ions by OH- ions. Dissociation constant of an acid (Ka) (or a base (Kb)A measure of the extent to which the acid (or base) dissociates (ionises) in solution, and which is given by Ka = . It is only applicable to weak acids and bases.Distilled waterWater which has been boiled and the steam from it condensed; it therefore contains no dissolved solidsDouble bondTwo shared pairs of electrons.Dynamic equilibriumSee EquilibriumEffluentLiquid industrial wasteElectrochemical seriesA list of elements in decreasing order of ease with which they lose electrons.ElectrodesThe conductors by which the current enters and leaves the solution in electrolysis. The positive electrode is called the anode and the negative electrode is the cathode.ElectrolysisThe process in which an electrolyte is decomposed by passing a current through it.ElectrolyteA compound which in liquid state conducts electricity and is decomposed in doing so.ElectronegativityThe relative attraction that an atom in a molecule has for the shared pair of electrons in a covalent bond.ElectroplatingThe process of covering one metal (usually a cheap and/or plentiful one) with a thin layer of a more expensive one, to provide protection and/or better appearance.ElectropositiveDescribes elements that readily lose electrons to form positive ions.ElementSubstance made up of one type of atom – Can’t be broken into anything simpler by chemical meansElimination ReactionReaction where a small molecule is produced when two molecules join to make a bigger molecule. E.g. Acid plus alcohol = ester plus waterEmpirical formulaThe formula of a compound that shows only the ratio in which the different types of atom are present in the molecule.Endothermic reactionA reaction in which heat is taken in or “used up”. Feels cold.Energy LevelThe fixed energy value that an electron in an atom may have.EquilibriumA balanced state of constant change in a system. Chemical equilibrium refers to the state in a reversible reaction when the rate of the forward reaction is equal to the rate of the backwards reaction.Equilibrium constant, KcFor a reversible system, a constant that tells how far the reaction has gone to the right-hand side, by the time equilibrium has been established. For the system: A + BC + D, the equilibrium constant is given by: K=EsterificationA reaction in which an acid and alcohol react together, to form an ester and water.EutrophicationThe enrichment of water with nutrients, which leads to excessive growth of algae.Excited stateThe state of an atom when one or more of its electrons have been promoted to higher energy levels.Exothermic reactionA reaction in which heat is liberated. Feels hot.FeedstockRaw materials needed for a chemical manufacturing process.Fixation of nitrogenThe process in which atmospheric nitrogen is made to combine with other elements to form useful compounds.FlocculationThe process in which small particles in water are made to coagulate (clump together) and form a precipitate. It is done by adding a FLOCCULATING AGENT such as aluminium sulphate or iron (III) sulphate.Fractional distillation (or fractionation)The process in which a mixture of liquids is separated into components or fractions of different boiling points (or different boiling points ranges).Functional groupA group of atoms on which the characteristic properties of a particular compound depend.Gay Lussac’s lawIn a reaction between gases, the volumes of the reacting gases and the volumes of the products, if gaseous, are in the ratio of small whole numbers (at the same temperature and pressure).General gas lawThis is a combination of Boyle’s and Charles’ laws, and is most simply expressed as P1V1/T1 = P2V2/T2Ground stateThe state of an atom when all of its electrons are in their lowest available energy levels.HalogensThe elements of Group 7 of the Periodic Table (i.e., fluorine, chlorine, bromine and iodine).Half-lifeThe time taken for half of the nuclei in any given radioactive sample to decay.HalidesCompounds which contain a halogen and one other element (e.g., fluorides, chlorides, bromides and iodides).Hard waterHard water is water which will not readily form lather with soap due to the presence of dissolved calcium or magnesium salts in the water.Heat of combustionThe heat produced when one mole of a substance is completely burnt in excess oxygen. Heat of formationThe heat change which occurs when one mole of a compound is formed from its elements in their standard states.Heat of reactionThe heat change which occurs when the reaction takes place according to a given chemical equation.Heat of neutralisationThe heat change that occurs when one mole of H+ ions from an acid reacts with one mole of OH– ions from an alkali.Heisenberg’s uncertainty principleIt is not possible to ascertain both the position and the momentum of an electron in an atom simultaneously.Hess’ lawThe heat change for a given reaction depends only on the initial and the final states of the system, and is independent of the path followed.Heterogeneous catalysisA system in which there is a distinct boundary between the catalyst and the reactants.Heterolytic Fission Splitting of molecule to give 2 different species. E.g. splitting of Br2 to Br+ and Br- when reacting with an alkene. Also called Ionic Addition.Homologous Series: A series of compounds, all members of whichHave same General Formula.Same Functional GroupEach member differing from the previous member by a (CH2) unit.Showing gradations in physical properties.Similar chemical propertiesEach member having a similar method of preparation.Homolytic FissionThe breaking of a covalent bond so that each atom retains one of the shared electrons. Splitting to give two identical radicals. e.g. Cl2 = 2 Cl. When reacting with alkanes or other saturated compounds Also called Free Radical SubstitutionHomogenous catalysisA system by which both the catalyst and the reactants are in the same phase or state.Hund’s ruleWhen two or more orbitals of equal energy are available to electrons, the electrons occupy them single before filling them in pairs.HydrolysisThe decomposition of a compound by means of water (or the OH– ion), the water also being decomposed in the reaction.HydrocarbonsCompounds containing hydrogen and carbon only. Hydrogenation The reaction in which hydrogen adds on across a double or triple bond.HydrationA reaction in which a compound combines with water.HydrolysisBreaking up a large molecule by inserting the components of water into it. Hydrogen bondThe electrical attraction between the slightly positive hydrogen atoms of one molecule and slightly negative atom in another molecule.Ideal gasOne that obeys the gas laws at all temperatures and pressures. In reality no such gas exists.Ideal gas equationpV = nRT, where p is the pressure (in pascals (Pa)), V is the volume (in m3), n is the number of moles, R is the general gas constant, T is the temperature (in Kelvin).IndicatorA substance, which by means of a colour change, indicates the presence of another substance.IonAn atom or group of atoms which has either lost or gained electrons and has therefore become either positively or negatively charged.Ionic bond Bond involving transfer of electrons. Lose electrons = plus; gain electrons = minus charge. Occurs when difference in electronegativity is greater than 1.7Ionic product of waterThe product of the H+ and the OH – concentrations in water (and all aqueous solutions), or, Kw = [H+] [OH–].Ionisation energyThe first ionisation energy of an element is the minimum energy required to completely remove the most loosely-bound electron from a neutral gaseous atom in its Ground State. [ X – e- = X+ ]The second ionisation energy is the energy required to completely remove the second most loosely bound electron from the ion X+ – e- = X2+IsomersCompounds having the same molecular formula but different structures.IsomerisationA process in which straight-chain hydrocarbons are heated under pressure with a catalyst, when they form branched-chain hydrocarbons.IsomerismThe existence of compounds which have the same molecular formula but different structural formulae. The different compounds are called isomers.IsotopesAtoms of the same element but which differ in the numbers of neutrons in their nuclei (or, have different mass numbers).Kilogram Calorific valueThe quantity of heat liberated when 1 kg of a fuel is completely burnt.KnockingPremature ignition of fuel in petrol engineLaw of conservation of energy (or first law of thermodynamics)This states that energy can neither be created nor destroyed.Le Chatelier’s principleIf a system in equilibrium is subjected to a stress, then the system will alter so as to oppose the effect of the stress.Limiting ReactantReactant not in excess. Controls how much product can be produced.Lone pairPair of electrons in the outer shell of an atom not involved in bonding.Mass numberThe number of protons and neutrons in an atom of the element.MatterAnything that occupies spaceMetalAn element that ionises to give positive ions.Metal oreA mineral that contains a compound of the metal and from which the metal is extracted.Melting pointThe temperature at which a solid changes to a liquid.MolarityThe concentration of a solution in moles per litre. (Mol/L)Molar solutionA solution that contains one mole of the solute per litre (dm3) of solution.Molar volume of gasThe volume occupied by one mole of the gas. It is equal to 22.4 litres at s.t.p. and about 24 litres at room temperature.Mole [mol]That amount of a substance which contains the Avogadro Constant number of particles. The Mole = RMM (relative molecular mass) in grams = Avogadro’s number (6 x 1023)= 22.4l of any gas at STP [ 273 K and 760mm of Hg ]MoleculeSmallest particle of substance that shows properties of that substanceGroup of atoms chemically joinedMolecular formulaThe molecular formula of a compound indicates the number of atoms of each kind in the Nitrogen fixationConversion of atmospheric nitrogen to nitrogen compounds that can be used. (useful compounds, chemically reactive)Negative catalyst (or inhibitor)A substance that decreases the rate of a chemical reaction.Noble gasOne of the elements of Group 0 of the Periodic TableOctane NumberMeasure of the tendency of the fuel to resist knocking. Heptane = 0; 2,2,4 trimethylpentane = 100Octet RuleThis states that when atoms bond together, they attain the stable structure of having eight electrons in the outer shell of their atoms. However, the Octet Rule is nothing more than just a guide and there are many exceptions to it.OrbitalThe region in space around the nucleus of an atom in which the electrons are most likely to be found.OxidationA process in which there is loss of electrons from an atom or ion.Oxidation numberThe charge which an atom of the has, or appears to have, when electrons are distributed according to certain rules.Oxidising agentA substance that causes oxidation. The oxidising agent takes the electrons and is therefore reduced in the process (since it gains the electrons).OxygenateA fuel that has a very high octane number because of the presence of some oxygen in it.Pauli’s exclusion principleNo more than two electrons can occupy an orbital, and this they can only do if they have opposite spin.PeroxideAn oxide in which contains two oxygen atoms joined to each other.Permanent hardnessThe type of hardness which cannot be removed by boiling the water; it is caused by dissolved sulphates and chloridespHMinus the log (to the base 10) of the H+ concentration, or, pH = -log10 [H+]. pH is thus a measure of the acidity or alkalinity of a solution.Pi bondA covalent bond made by the sideways overlapping of two p orbitals. The second bond in a double bond is a pi bond.pOHMinus the log10 of the OH–concentration. The sum of the pH and the pOH of any aqueous solution is 14.Planar CarbonCarbon with a double or triple bond to another carbon. No rotation possible around bondPlasticA material which can be softened and moulded by heat and pressure.Polar Covalent BondBond involving sharing of pairs of electrons – one from each atom – not shared equally – one with highest electronegativity is δ- other is δ+. Occurs when difference in electronegativity is less than 1.7PolymerA substance formed by the combining together of large numbers of small molecules.PolymerisationA process in which many small and simple molecules (called monomers) join together to form a much larger molecule, called a polymer.Primary Standard SolutionA substance which can be obtained in a stable, pure and soluble solid form so that it can be weighed out and dissolved in water to give a solution of accurately known concentration.Principle quantum numberA number that indicates the main energy level, or shell number, of an electron in an atom.ProductA substance formed in a chemical reaction.PromoterA substance that improves the activity of a catalyst, but which itself is not a catalyst.Pure covalent bondA covalent bond in which the pair of electrons is shared equally between the two joined atoms.RadioactivityThe spontaneous breaking up of unstable nuclei with the emission of one or more types of radiation.Rate of a reactionThe change in the amount (or of the concentration) of reactant (or product) per unit time.RecrystallisationA process used to purify substances. The impure substance is dissolved in a suitable solvent, the solution filtered and allowed to crystallise.Reducing agentA substance that causes reduction. The reducing agent supplies the electrons and is therefore oxidised in the process (since it loses the electrons).ReductionA process in which an atom or ion gains electrons.Relative Atomic Massaverage of the mass numbers of the isotopes of the element.as they occur naturallytaking their abundances into accountexpressed on a scale on which atoms of the carbon 12 isotope have a mass of exactly 12 units.Relative Molecular Mass (Mr)The sum of the relative atomic masses of all the atoms in a molecule of the compound.The mass of one molecule of that compound compared with one twelfth of the mass of the carbon 12 isotope.Mass of one mole of a compound = Relative Molecular Mass in grams.Reversible reactionA reaction that can go in either direction (i.e., from left to right, or from right to left).SaltThe substance formed when the hydrogen of an acid is replaced by a metal.SaponificationA hydrolysis of an ester carried out in the presence of alkali. (Saponification of natural fats and oils yields soaps and glycerol.)SaturatedContains only single covalent bonds. Does NOT decolourise bromineSigma bondA covalent bond made by the end-on overlapping of two p orbitals. A single covalent bond is a sigma bond.Soft waterSoft water is that which will readily form a lather with soap because there are no calcium or magnesium salts present.Specific heat capacity formulaE = m c , where E (in joules) is the heat energy required to heat a substance of mass m and specific heat capacity c, through a temperature of .Standard solutionA solution whose concentration is known.Standard temperature and pressure (S.T.P)273 K or 0 °C and 101 kPa (101 x 103 Pa).Steam reformingA reaction used for the manufacture of hydrogen, by reacting steam with natural gas at high temperature.Strong acidAn acid that is fully dissociated in solutionStructural formulaThe formula of a compound showing the way in which the atoms are arranged in the molecule.Sub-level An atomic sub-level is a sub-division of a main energy level consisting of one or more orbitals of the same energy / discrete energy values available in main level (shell)Substitution reactionA reaction in which an atom or group of atoms in a molecule is replaced by other atoms or groups of atoms. Substitution reactions are characteristic of the alkanes.Temporary hardnessThe type of hardness which can be removed by boiling the water; it is caused by dissolved hydrogencarbonates. These salts are decomposed on being heated (forming the corresponding carbonate – which is precipitated).Tetrahedral CarbonCarbon with only single bonds to other carbons or atomsTransition MetalMetal that forms at least one ion with a partially filled d sublevel. May have coloured ions. Variable valency and catalytic propertiesTriple bondThree shared pairs of electrons.UnsaturatedContains at last one double or triple covalent bond. Does decolourise bromineValence Shell Electron Pair Repulsion Theory [VSEPR]Pairs of electrons repel each other due to like charges degree of repulsion depends on type. Accounts for shapes of moleculesValencyThe number of electrons which an atom of an element must either gain or lose to attain a noble gas structure. Combining power of an atomVolatile Liquid A liquid that changes quickly and easily to a vapour.VoltameterA vessel fitted with electrodes in which electrolysis takes place. (Do not confuse voltameter with voltmeter, which is an instrument to measure voltage.)Weak acidAn acid that is only slightly dissociated in solution. ................
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